A rechargeable battery is an electrochemical cell that stores energy, delivering that energy upon discharge of current based upon the demand of the electrical device. A rechargeable battery may be recharged by forcing an electrical current through the battery in a direction opposite to that of discharge.
A commonly encountered problem with rechargeable batteries is a loss in the energy capacity of the battery over subsequent recharging cycles resulting in a reduced amount of time of battery usage until the next recharging cycle. For example, a loss in the ability to retain full energy capacity of a battery may result after a charging cycle follows a period of use when the battery does not become fully discharged. The loss in the ability to retain full energy capacity may become exasperated when there are repeated cycles of shallow discharging followed by a charging cycle. To reduce the extent of loss to retain substantially full energy capacity of a battery further preventing a rapid deterioration in available energy capacity after a charge cycle, manufacturers recommend subjecting a rechargeable battery to a deep discharge prior to recharging the battery.
While there are many phenomena that can contribute to this loss in ability of the battery to retain full charge capacity, it is known that a deterioration in the ability of an active constituent to become regenerated at any one or both of the anode and cathode may be a contributing factor. For example, it has been reported that the decline in capacity of lead acid batteries is associated with a progressive change in the nature of the active materials of the cathode and the anode, which also contributes to a reduction in life of the battery as well as a loss in the ability of the battery to retain capacity. The initial state of the surface structure of the cathode and anode is porous allowing a greater amount of the active material to become exposed to the surrounding electrolyte of the battery. As the battery undergoes multiple discharge and recharge cycles, the surface structure of the cathode and anode progressively becomes increasingly defined by aggregate crystalline structures that reduce the overall surface contact of the active material with the electrolyte solution of the battery.
Attempts in the prior art to reduce these effects in a battery have been directed to improved battery charge cycles that include insuring the battery becomes deeply discharged prior to recharging the battery to a recommended operating level. Other battery chargers in the prior art control the pattern of charge and, in some cases, may include a slight discharge sequence over the period of charging the battery. For example, U.S. Pat. No. 5,633,574 to Sage discloses a charging sequence for a battery that includes repeatably applying a sequence that includes 1000 milliseconds of charging, 2 milliseconds of no charging, 5 milliseconds of discharging, and 10 milliseconds of no charging may reduce the extent of loss in ability for the battery to retain full charge capacity. U.S. Pat. No. 5,998,968 to Pittman et al. discloses applying a discharge, charge, and rest period to a battery in a predetermined charging sequence until the battery becomes fully charged. U.S. Pat. No. 5,777,453 to Imanaga represents even another charge sequencing strategy whereby voltage pulses are periodically applied to a battery followed by a rest period when no voltage is applied during the charging sequence.
Repeated losses in the ability of the battery to retain full charge capacity over multiple charging cycles may also contribute to an overall reduction in the life of the battery. I.e., it is known that a loss in the ability of the battery to retain capacity is not fully irreversible and may be cumulative over the life of the battery resulting in an overall reduction in the life of the battery.
During a charge cycle, the electrodes or plates attract ions—negative ions to the positive plate and positive ions to the negate plate—which impedes the further transfer of ions to the plates. As the battery becomes charged, an increased impedance develops resulting in an increased resistance of the battery to become charged. Eventually, upon completion of charging and removal of any overvoltage, an equilibrium will develop at the anode and cathode such that the rate of transfer of ions to the electrodes equals the rate of transfer of the same types of ions away from the electrodes.
The equations of Boltzmann, represented by equation 1, and Nernst, represented by equation 2, describe the thermodynamic equilibrium (the stable state) that develops in an electrochemical system in terms of the ratio of the density of ions in the bulk electrochemical solution, Dse, relative to the density of the same types of ions present in the surface layer of the electrode, Dme, in relation to the potential difference, (Vse−Vme), that exists between the electrochemical solution and the electrode and its mutual dependence on said ratio Dse/Dme. See, e.g., Christian Gerthsen and Helmut Vogel: Gerthsen Physics, 19 ed., Springer Verlag, Berlin and New York.
                              (                                    D              se                                      D              me                                )                =                  ⅇ                                    -                              (                                                      V                    se                                    -                                      V                    me                                                  )                                      *                          q              kT                                                          (        1        )                                          (                                    V              se                        -                          V              me                                )                =                              -                          (                              kT                q                            )                                *                      ln            ⁡                          (                                                D                  se                                                  D                  me                                            )                                                          (        2        )            where:
q=charge of an electron, Coulomb
k=Boltzmann constant, Joule/Kelvin
T=absolute temperature, Kelvin
Dse/Dme=ratio of the ionic density of the electrochemical solution to the ionic density of the surface layer at the electrode at equilibrium
(Vse−Vme)=potential difference between electrochemical solution and electrode at equilibrium, volts
At equilibrium conditions, the system is stable, i.e., the formation, growth or dissolution or phase transitions do not occur. At equilibrium, the flux of any ionic species into the surface layer at the electrode will be compensated for by the flux of an equal number of the same ionic species from the surface layer at the electrode into the electrochemical solution.
In all chemical systems there is a tendency to change to the equilibrium state. See, e.g., James E. Brady: General Chemistry—Principles and Structure, John Wiley & Sons, New York. If an existing equilibrium is disturbed, for example, by imposing a change in the potential at the electrode, then the ratio of the ionic density of the electrochemical solution to ionic density of the surface layer at the electrode will change until a new equilibrium condition is achieved. The relaxation time is defined as the amount of time needed for the system to arrive at a new equilibrium condition. The relaxation time constant, which characterizes the change in ratio of ionic densities versus time, is defined by the specific dielectric constant divided by the specific electrical conductivity, both of which are properties of the electrolytic solution.
Favorable conditions for phase transitions, i.e., for ions from the electrolyte solution discharging on the surface of the electrode, occur when the solution is supersaturated and the system departs from its equilibrium condition. For example, supersaturation occurs when the potential Vs of the ions in the electrochemical solution is greater than the equilibrium potential Vme on the electrode, as represented by equation (3).(Vs−Vme)>0  (3)
There are two possibilities for addressing this supersaturation condition. One possibility is to impose a potential on the electrode Vm that is more negative or less than the potential of the electrode at equilibrium Vme while the potential of the electrochemical solution is maintained at its equilibrium potential as represented by equation (4).(Vse−Vm)>0  (4)
The difference between the potential of the electrode at equilibrium and the potential of the electrode under the circumstances as described above is known as electrochemical over-potential or the electrochemical overvoltage as represented by equation (5).(Vme−Vm)>0  (5)
Another possibility for addressing the supersaturation condition is by imposing on the electrochemical solution a potential Vs that is higher than the potential of the electrochemical solution at equilibrium Vse by keeping the potential on the electrode Vm at its equilibrium potential Vme. Thus, the circumstances of the overvoltage condition as represented in equation (3).
The two quantities, the condition of supersaturation and the overvoltage, can be considered as measures for the deviation from the state of stable thermodynamic equilibrium. However, the mere fact that the system is supersaturated and the overvoltage exists does not necessarily create a phase transition. Rather, these conditions increase the probability that a phase transition may occur. See, e.g., Alexander Milchev: Electrocrystallization—Fundamentals of Nucleation and Growth, Kluwer Academic Publishers, New York.
There remains a need in the art for an apparatus and method that operates to reduce the loss of the capability of the battery to store energy over time and increase the overall life of the battery during the entire operational cycle of the battery, i.e., even outside the period when the battery is being charged.